Respiration substrates

In respiration, substrate organic molecules containing carbon-hydrogen bonds (food) and oxygen are absorbed by prokaryotic cells or by the mitochondria of eukaryotic cells. The oxygen reacts with electrons that are derived from metabolic changes to the carbon-hydrogen bonds of the substrates. The final steps of substrate metabolism, the Krebs cycle,... 

Isolated axes of peas are not dependent on the reserve material of the cotyledons during the initial stages of radicle elongation [17]. Reserves of carbohydrate, protein and fat in the radicle itself must be sufficient for these early events, and sucrose, raffmose and stachyose probably serve as sources of respirable substrate. But after these early events have passed, the further development of the root and shoot systems depends upon the contributions from the cotyledons. Their stored carbohydrate (and the other reserves) are hydrolysed and transported into the axis. It is not yet clear if the initiation of mobilization is actually controlled by the embryonic axis this is discussed fully in Chapter 7. We can say, however, that the subcellular changes in the cotyledons that precede and accompany reserve mobilization do need the presence of axis, at least over the first 48 h after the start of imbibition. 

The ready reversibility of this reaction is essential to the role that qumones play in cellular respiration the process by which an organism uses molecular oxygen to convert Its food to carbon dioxide water and energy Electrons are not transferred directly from the substrate molecule to oxygen but instead are transferred by way of an electron trans port chain involving a succession of oxidation-reduction reactions A key component of this electron transport chain is the substance known as ubiquinone or coenzyme Q... 

Oxygen is used in these microbiolreactions to degrade substrates, in this case organic wastes, to produce energy required for ceU synthesis and for respiration. A minimum residual of 0.5 to 2.0 mg/L DO is usually maintained in the reactors to prevent oxygen depletion in the treatment systems. 

Respiratory, or oxidative, metaboHsm produces more energy than fermentation. Complete oxidation of one mol of glucose to carbon dioxide and water may produce up to 36 mol ATP in the tricarboxyHc acid (TCA) cycle or related oxidative pathways. More substrates can be respired than fermented, including pentoses (eg, by Candida species), ethanol (eg, by Saccharomjces), methanol (eg, by Hansenu/a species), and alkanes (eg, by Saccharomjces lipoljticd). 

When excess substrate interferes with growth and/or product formation. One example is the production of baker s yeast. It is known that relatively low concentrations of certain sugars repress respiration and this will make the yeast cells switch to fermentative metabolism, even under aerobic conditions. This, of course, has a negative effect on biomass yield. When maximum biomass production is aimed at, fed batch cultures are the best choice, since the concentration of limiting sugar remains low enough to avoid repression of respiration. 

In practice, carbon limited chemostat cultures are used to estimate the P/O quotient These conditions are used because they favour the most efficient conversion of the carbon substrate into cellular material, ie the highest efficiency of energy conservation. The steady state respiration rate (qo,) is measured as a function of dilution rate (specific growth rate) and Yq can be obtained from the reciprocal of the slope of the plot. qo, is also known as the metabolic quotient for oxygen or the specific rate of oxygen consumption. 

Many inhibitors of substrate oxidations, substrate transport, electron transport, and ATP synthesis are known including many well-known toxins (see Sherratt, 1981 Harold, 1986 Nicholls and Ferguson, 1992). These are not discussed here except to mention specific uncouplers of oxidative phosphorylation. Classic uncouplers such as 2,4-dinitrophenol have protonated and unprotonated forms, both of which are lipid soluble and cross the inner mitochondrial membrane discharging the proton gradient. This prevents ATP synthesis and stimulates respiration. 

Burmell, P. L., Tait, D. E. N., Flanagan, P. W. and Van Cleve, K. (1977). Microbial respiration and substrate weight loss, I. A general model of the influences of abiotic variables. Soil Biol. Biochem. 9, 33-40. 

Padmanabhan P, S Padmanabhan, C DeRito, A Gray, D Gannon, JR Snape, DC Tsai, W Park, C Jeon, EL Madsen (2003) Respiration of C-labeled substrates added to soil in the field and subsequent 16S rRNA gene analysis of C-labeled soil DNA. Appl Environ Microbiol 69 1614-1622. 

P. Hogberg and A. Ekblad, Substrate-induced respiration measured in situ in a C3-plant ecosystem using additions of C4-sucrose. Soil Biol. Biochem. 2 1131 (1996). 

Polarographic studies of a mitochondrial fraction from Hymenolepis diminuta showed that of four substrates tested, DL-glycerol-3-phosphate was the most rapidly oxidized, but the highest respiratory control ratio (1.7) was obtained with dl-isocitric acid. With isocitrate as substrate oxyclozanide at 1.61 nM stimulated O uptake and relieved oligomycin inhibition of adinosine diphosphate-stimulated respiration, but at concentrations above 2 pM progressively inhibited O uptake. Rafoxanide, niclosamide, 3,4,5-tribromo-salicylanilide, nitroxynil, resorantel, di-chlorophen, and 2,4-dinitrophenol exhibited effects similar to those of oxyclozanide on the respiration in cestode mitochondria. The relative potencies were compared and the possible mode of action discussed [38]. 

Historically, measurement of the microbial biomass has been a tedious, time-consuming occupation involving staining and direct counting or use of culture media and enumeration of individual microbial communities. However, in the last 20 years, a suite of methods have been developed for more rapid assessment of the microbial biomass. These include the substrate-induced respiration method (Anderson and Domsch 1978), the chloroform fumigation-incubation method (Jenkinson and... 

Certainly, calculation of the metabolic quotient can reveal trends very different from those of basal respiration. As shown in Fig. 1, for the 7 land uses, trends in basal respiration were broadly similar to those for microbial biomass C and organic C. However, when the metabolic quotient was calculated, trends with land use were very different. Values were greater under sugarcane, maize and to a lesser extent annual ryegrass, than the other treatments. This suggests that the microbial community under these arable land uses is under more stress and/or has a different composition to that under the others. The most likely microbial stress under these land uses is likely to be a shortage of available substrate C. 

Metabolic quotient Variable often in the range of 2-40pg C02-C mg-1 biomass day-1 Basal respiration per unit of microbial biomass C. Provides a useful measure of the efficiency with which microbes are using substrate C. Often used as an index of the degree of microbial stress. 

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